After the radiator upgrade, did you make sure the cooling system was fully filled before driving the car?
After this last drive when the engine began running oddly, did you pull the spark plugs? As already mentioned, used plugs tell a story.
Since the engine still starts and runs, does it make any odd noises? Even though the temperature gage doesn’t work, you can still get an idea of how “hot” it is getting, put your hand on the thermostat housing, upper and lower radiator hoses, the heater hoses, and the upper radiator tank. If it is too hot to hold your hand on it, it is running too hot. Otherwise, overheating may not be the issue.
It would also be a good idea to use a cooling system pressure tester. Pressurizing the system will immediately show any leaks.
I can’t tell by the picture you posted where the coolant is leaking from, so try and get some better pictures to post. All we can see is a wet spot on the crossmember. Also, it looks like the hoses are original. When you changed out the radiator, it would have been a good idea to replace all the hoses.
You mentioned the weather has been poor, do you not park the car inside? Working on any car outdoors sucks, and these cars need to be kept inside anyway.